LIGO Caltech News

- Contributed by Riccardo DeSalvo

On February 9th 2000, two large crates, each containing
a Seismic Attenuation System (SAS) tower designed to the
requirements of the TAMA interferometers, left the Caltech
Synchrotron Laboratory en route for Japan. The shipment
arrived there safely five days later, ready for installation.
Soon after, Akiteru Takamori and Riccardo DeSalvo arrived
also to install the two towers in the vacuum chambers of the
three-meter Hongo campus interferometer.

This was a delivery whose process began in Autumn 1999, when
TAMA
scientists realized their seismic attenuation might be
improved in the future with stacks of the SAS type. This would
facilitate reaching better sensitivity, and the very low
frequency isolation promised that the interferometer lock
could be maintained for longer stretches of time, even during
noisy periods of the day.

In January 2000, Seiji Kawamura of TAMA suggested that the LIGO
SAS Group collaborate with TAMA in this research. While a quick
study, using commercial active systems, was already planned, the
SAS appeared to be a more robust solution to TAMA's longer-range
R&D goals. SAS group expertise was seen as an advantageous way
to move rapidly forward.

The SAS group, led by Riccardo DeSalvo, quickly agreed to adapt
an existing design for advanced LIGO to the TAMA requirements.
From the LIGO point of view, the project was viewed as a mature
test-bed for the LIGO SAS technique. The project was to become
Akiteru Takamori's PhD thesis.

Pre-prototype tests and design began immediately. Production was
to start in Summer 2000, and completion of two seismic attenuation
chains were scheduled by the fall. In July and August 2000, while
starting the adaptation of the seismic towers intended for the
mirror suspension payload, Akiteru realized that an unacceptable
mismatch was present between the existing TAMA suspensions and the
TAMA-SAS already in production. He quickly produced a fresh proposal
to redesign new suspensions and have them built together with the
SAS, with only a small cost in schedule delay. Akiteru's plan was
approved by the TAMA collaboration, and by September 2000 the new
mechanical suspension redesign was completed and ready for production.

At right: A schematic overview of the TAMA SAS towers.

The SAS towers are based on the LIGO-developed Monolithic Geometric
Anti-Spring Filters (MGASF), each capable of 60 dB attenuation. The
TAMA-SAS towers also incorporated the
Virgo inertial damping of the
attenuation chain resonant modes, as well as the advanced accelerometers
designed as part of the PhD thesis of Alessandro Bertolini and provided,
in part, by the Univesita' di Pisa. The new SAS/SUS system is expected
to deliver a few tens of nanometers of r.m.s. mirror motion below the
observation frequency range. In addition, it provides passively
attenuated residual seismic noise crossing the thermal noise level at
or below 10 Hz. The control electronics are adapted from the Virgo
DSP system.

The new suspensions, fully compatible with the SAS isolation towers,
were based on a double pendulum with recoil mass mirror controls first
developed by Virgo, along with the multiple pendulum passive mode damping
developed and tested by TAMA scientists, in addition to the Caltech
developed MGAS springs for low frequency vertical resonances.

Shown below: (1) The TAMA SAS assembly lab. (2) A top view of one
of the towers. (3) An inner view.

The suspension production contract was awarded in October to Italy's
Galli and Morelli, who were already finishing the SAS towers. In December
2000, Riccardo, Alessandro and Akiteru took delivery in Lucca, Italy from
Galli and Morelli of the completed SAS/SUS mechanics. The SAS part was
fully assembled and tuned at Caltech in January 2001, and in the first
week of February the equipment was packaged in the two large, protective
crates for shipment to Japan.

Equal to the technical challenges were the complexity of the international
purchasing, integration of the subsystem, as well as the transport and
proper entry into Japan. It was thanks to the ingenuity of Irena Petrac
of Caltech, and Seiji Kawamura (a TAMA scientist who transformed himself
into an administrator for the occasion) that all these challenges were met.

In the days between January 19th and 23rd, Riccardo and Akiteru, helped
by Professor Kimio Tsubono, Kazuhiro Yamamoto, and others, unpacked the
crates, installed, aligned and then tuned the two towers in the freshly
arrived three-meter interferometer vacuum tanks. By March 3rd one of the
Suspensions was already assembled and suspended from a SAS tower. In an
already famous, perfectly summarizing comment, Akiteru stepped back,
surveyed the arrangement and said conclusively, "Well, it looks just
like the drawings!"

Above: (1) Beginning installation of a tower
at TAMA. (2) Both towers now visible. (3) Akiteru smiles in satisfaction
during assembly of the double pendulum. (4) Double pendulum assembly,
but where's Riccardo?

In the next five months, Virginio Sannibale, Giovanni Losurdo, Szabi Marka
and Alessandro Bertolini will assist in finishing the two systems with all
their wiring, mirrors, instrumentation and tuning, so as to complete a
Fabry Perot interferometer test. This test in the three-meter interferometer
is expected to produce the final validation of the TAMA-SAS/SUS system.
TAMA will then be able to consider this system for use in the pioneering
TAMA 300 interferometer itself.

This effort was a true international scientific collaboration between almost
all of the major gravitational-wave interferometer groups, Virgo, LIGO and TAMA.
It is a precursor of even wider collaborations between all gravitational-wave
experiments.

To view a complete viewgraph presentation of the the TAMA SAS story in .pdf
format,
click here.
To read the TAMA SAS technical document in .pdf, containing detailed scientific
results,
click here.